Because unless you want every company to be a random Amazon brand or initialism, that's how it kinda has to be, and it works fine until one company gains so much market share the word starts being associated with only them.
Think of like, Target or Shell. Both are huge companies, but their fields are narrow. You might confuse a Target named restaurant or pharmacy to be the Target, but probably not much more. And if it doesn't have anything to do with oil or gas, it's almost certainly not that Shell.

Apple is just so huge I wouldn't be surprised if at this point people think of iPhones while buying lunch. And even they started as "Apple Computers, inc", because they wouldn't have gotten just "Apple" if they had tried.